The purified terephthalic acid is produced by oxidizing paraxylene with a molecular oxygen in liquid phase, dissolving a crude product of terephthalic acid in hot water, and catalytically hydrogenating the aqueous solution of the crude terephthalic acid by contacting with a catalyst of noble metal belonging to the group VIII of the Periodic table in the presence of hydrogen. This production process employing an apparatus of commercial scale has a production performance ranging many years.
In this process, the slurry prepared by cooling the hydrogenated reaction liquid and including the precipitated terephthalic acid crystals is separated to the first crystallization products and the first mother liquor product usually at the temperature of from 120° C. to 220° C. The first mother liquor product contains considerable amounts of dissolved impurities such as paratoluic acid being a hydrogenated product of 4-carboxybenzaldehyde (4CBA), which is a main impurity in the crude terephthalic acid, the terephthalic acid corresponding to the solubility of the isolation temperature and other impurities, etc. Further, it may contain small amounts of the terephthalic acid crystals.
Mere disposal of the first mother liquor product significantly increases the load of an effluent treatment. Further, it is rational that the paratoluic acid and the terephthalic acid are recovered as the second crystallization products and reused because the paratoluic acid is a precursor of the terephthalic acid. The production of the purified terephthalic acid containing recovery and return of the second crystallization products to the liquid phase oxidation process is described in Japanese Examined KOKOKU Patent Publication No. Shou 56-35174 and Japanese Laid-Open Patent Publication No. Hei 10-195016, etc.
Additionally, Japanese Laid-Open Patent Publication No. Hei 8-231465 discloses a mother liquor substitution method for dividing terephthalic acid crystal particles instead of using elevated pressure centrifugal separation method.
Regarding the recovery of the second crystallization products to the liquid phase oxidation process, the foregoing Japanese Examined KOKOKU Patent Publication No. Shou 56-35174 further teaches that “The crystallization temperature of the second crystallization products should be 130° C. to 100° C.”. Further, Japanese Laid-Open Patent Publication No. Hei 10-195016 teaches that “The raw water solvent discharged is subsequently cooled to 40° C. or less by decompression.”.
According to the experiences of the inventors ranging many years, however, when the slurry with comparatively low concentration of the crystals is generated in a crystallization vessel as the case of recovering the second crystallization products, crystals are easy to accumulate on the inner wall and the internal structure of the crystallization vessel. Further, when the concentration of the crystals in the crystallization vessel is low, a stable separating operation in downstream tends to become difficult in many cases because the particle sizes of the crystals relatively decrease.
In other words, the inventors tried to introduce the first mother liquor product separated at the temperature of about 145° C. into a crystallization vessel in the above-mentioned purified terephthalic acid preparing apparatus, to cool down to 100° C. with flash vaporization of a solvent (water), and to supply the generated slurry to a separating means for the second crystallization products, the internal structure of the crystallization vessel was found to be adhered and accumulated by the crystals. Furthermore, the pressurized filtration method selected as separating means was found to induce an intense blocking of a filter, and frequent cleanings of the filter with short intervals were necessary.
It is an object of the present invention to provide a way of long and stable operation of the crystallization vessel and the separating means of the crystals in the process of feeding the slurry solution of terephthalic acid to the crystallization vessel, precipitating the terephthalic acid in the crystallization vessel, generating and discharging the slurry including crystals and solvent.